1. Field of the Invention
This invention relates to corrosion-resistant double-walled tubes and processes for manufacturing such tubes. Double-walled tubes such as those produced by the process of the present invention are suitable for a variety of uses such as in automotive brake lines. This invention also relates to a process for brazing a selected metal to a non-ferritic steel surface.
2. Discussion of the Relevant Art
Safety standards in the automotive industry dictate that critical elements such as automotive brake lines be resistant to leakage, puncture and corrosion. In order to achieve these objectives, double-walled tubes for brake lines have been adopted as the industry standard. Such a double-walled tube consists of at least two thicknesses of a breakage-resistant metal material having sufficient properties to withstand fatigue due to prolonged vibration. The double-walled tube employed in automotive vehicles is generally referred to as "seamless", meaning that there is no seam extending the radial length of the tube wall, from the inner diameter of the tube to the outer diameter. Thus, the possibility of leakage at any joined seam is essentially eliminated. The material of choice, up to this point, has been carbon steel due to its inherent flexibility, strength and mechanical durability. A suitable brazing material such as copper or various copper alloys is plated over the surface of the carbon steel prior to formation to permit the ultimate formation of a seamless joint.
A drawback of carbon steel is its susceptibility to corrosion. In an attempt to eliminate this problem, carbon steel brake line tubes have been plated with a variety of corrosion-resistant materials, the most common of which is zinc. Zinc is plated onto the carbon steel surface of the brake line tube in a post-manufacturing process. In order to achieve sufficient corrosion resistance, plating thicknesses as great as 25 microns have been employed. Unfortunately, the zinc-plated surface is susceptible to cracks and chips due to road hazards and continued prolonged vibration. This leads to corrosion and, ultimately, to leaking of the brake line tube.
In order to alleviate this problem, zinc-plated carbon steel tubes have been further coated with high-strength polymers such as polyvinyl fluoride. Polyvinyl fluoride coatings can also crack and chip, and ultimately lead to corrosion problems. Additionally, brake lines coated with polyvinyl fluoride are difficult to dispose of or recycle once the vehicle has reached the end of its useful life.
Ideally, brake lines would be manufactured from a suitable, inexpensive non-corrosive material. However, corrosion-resistant metals such as nickel-chromium (stainless) steel are not amenable to double-walled tube manufacturing processes. Great difficulties have been encountered in imparting a copper overlay to a stainless steel surface, and it has been widely held that copper-plated stainless steel could not be successfully metallurgically brazed. Various face-centered cubic stainless steel materials are considered susceptible to liquid metal embrittlement due to diffusion of liquid copper into the grain boundaries. This can cause hot cracking which causes the cracks to be susceptible to propagation during subsequent cold forming steps. Steels of this type can exhibit embrittlement in weldments and their heat affected zones due to the face-center cubic grain structure when exposed to brazing temperatures in a conventional copper braze environment.
Additionally, the forming processes for manufacturing a continuous seamless double-walled tube require the use of lubricating compounds or materials, such as milling oils, which adhere to the surfaces of the metal and interfere with achieving a uniform 360.degree. metallurgical braze. Removal of these contaminants prior to brazing is imperative but difficult.
Thus, it would be desirable to provide a process for manufacturing seamless double-walled tubing in which contaminating lubricants can be removed or rendered harmless prior to metallurgical brazing. It is also desirable to provide a process in which a highly corrosion-resistant base metal such as non-ferritic or stainless steel can be successfully and economically employed. It is desirable to provide a process in which a selected metal alloy can be successfully metallurgically bonded to a non-ferritic steel surface. It is further desirable to provide a corrosion-resistant double-walled seamless tubing suitable for use in the manufacture of automotive brake lines; meeting the parameters of strength and durability commonly associated with such applications.
The present invention is a double-walled metal tube having an outer wall and an inner wall concentrically disposed therewithin. The inner and outer walls are both composed of steel having a face-centered cubic grain structure. The outwardly oriented surface of the inner wall is directed toward the inwardly oriented surface of the outer wall. The double-walled metal tube also has an intermediate bonding metal region interposed between the metallurgically bonded to the inner wall and the outer wall with the intermediate bonding metal region consisting essentially of a metal capable of metallurgically bonding with the inner wall and the outer wall.